Air composition every diver should know: oxygen about 20%, nitrogen about 79%, and carbon dioxide near 0.03%.

What are the three main gases in the air and their approximate percentages?

• A. Oxygen 20%, Nitrogen 79%, Carbon Dioxide 0.03%
• B. Oxygen 21%, Nitrogen 78%, Carbon Dioxide 1%
• C. Oxygen 25%, Nitrogen 74%, Carbon Dioxide 0.5%
• D. Oxygen 19%, Nitrogen 80%, Carbon Dioxide 0.01%

Answer: (A)

The three main gases in Earth's atmosphere are indeed oxygen, nitrogen, and carbon dioxide. Understanding their approximate percentages is crucial for divers, as these gases play significant roles in respiratory functions and the management of gas mixtures during diving activities. Oxygen makes up about 20% of the atmosphere, which is vital for respiration and is the gas we strive to maintain at optimal levels while underwater. Nitrogen, which constitutes approximately 79% of the air we breathe, is inert and does not support respiration, but it does dissolve in the blood and tissues and is a significant factor in the formation of nitrogen narcosis or decompression sickness during dives. Carbon dioxide is present in trace amounts, around 0.03%, and is a waste product of respiration that the body must expel to maintain proper acid-base balance. The other options contain inaccuracies in their values of these gases, making them less accurate representations of the atmospheric composition. The correct answer provides a clear and accurate view of the air's composition, which is essential for both general knowledge and safe diving practices.

Three gases, one life-supporting mix. That’s what you’re really breathing every time you stand at the edge of the pool or step onto a boat with your gear in hand. When we talk about the air around us, the numbers aren’t just trivia. They matter—especially when you’re planning to stay underwater for a while and want to keep things safe and comfortable.

The big three and their approximate percentages

Here’s the quick, practical breakdown you’ll see repeated in courses like IANTD Open Water Diver programs:

• Oxygen: about 20%

• Nitrogen: about 79%

• Carbon dioxide: about 0.03%

Yes, those numbers are approximate, and yes, they’re a touch dry to think about. But they’re exactly the kind of detail that shows up in the real world of diving. O2 is the gas your cells crave to keep you alive, N2 is the inert filler that carries you along, and CO2 is the waste product your body tries to vent out as you breathe. It’s a small balance with big consequences.

Why does this matter when you’re under water?

Let me explain with a quick, friendly reminder of how gases behave under pressure. At sea level, the air feels “normal” because the atmosphere is one atmosphere of pressure (1 atm). But when you descend, the surrounding pressure rises. Every meter of depth adds more pressure, and the gases you breathe are compressed along with you. That’s where the numbers above become a practical puzzle.

• Oxygen’s role: Oxygen is essential. In air, it’s roughly 20% of what you’re inhaling. Under water, the fraction in the gas you breathe doesn’t change, but your body experiences a higher ambient pressure. The result is a higher partial pressure of oxygen (PPO2). If PPO2 gets too high, you can get oxygen toxicity, especially at deeper depths. That’s why divers don’t simply crank up the oxygen percentage in mixed gases; they balance it to stay within safe PPO2 limits.

• Nitrogen’s role: Nitrogen is mostly inert with respect to your metabolism, but it’s not inert in the dive sense. It dissolves into your blood and tissues as you go deeper. This dissolving can contribute to nitrogen narcosis if you stay long or go deep, and it’s also involved in decompression considerations when you surface. In other words, the nitrogen portion of the air matters even though you’re not “using” it for metabolism.

• Carbon dioxide’s role: CO2 is a byproduct of your metabolism that your body must expel, especially during exertion. At depth, your lungs and chest muscles work a little harder, and any buildup of CO2 can make you uncomfortable, increase your breathing rate, or erode your sense of calm. That tiny 0.03% is enough to matter if ventilation is off or if you’re working hard.

Two common questions that pop up in studies about air composition

• “If the air is 20% oxygen, why doesn’t the oxygen level change with depth?” Great question. It’s still about the gas mix, but the key is the partial pressure. As you go deeper, the total pressure is higher, so the amount of O2 in your lungs (as a partial pressure) increases. The risk is too much O2 at higher depths, which can trigger oxygen toxicity. That’s why divers don’t simply increase the oxygen fraction in their breathing mix for every situation.

• “What about nitrogen narcosis?” Nitrogen doesn’t supply energy to your body the way oxygen does, but at depth the increased pressure makes nitrogen dissolve more into your tissues. That can have a narcotic-like effect for some divers—think of it as a safety valve you don’t want to overfill. It’s another reason why gas planning is essential, even with air.

A quick reality check against common answer choices

In many quizzes you’ll see something like this:

A. Oxygen 20%, Nitrogen 79%, Carbon Dioxide 0.03%

B. Oxygen 21%, Nitrogen 78%, Carbon Dioxide 1%

C. Oxygen 25%, Nitrogen 74%, Carbon Dioxide 0.5%

D. Oxygen 19%, Nitrogen 80%, Carbon Dioxide 0.01%

Why A is the better pick: the numbers align with the standard approximate composition of dry air at sea level. The “21% O2” and “~78% N2” you sometimes hear are rounding differences, but the CO2 is the telltale number—about 0.03% is the correct ballpark for ambient air. The other options tilt the balances in ways that don’t match everyday air, and that matters when you’re thinking about what you’re actually breathing during a dive.

From air to gas mixes: practical dives, not just theory

If you’re sticking with air for most recreational dives, you’re breathing roughly 21% oxygen and 79% nitrogen, with traces of CO2. But many divers experiment with gas blends to tailor a dive to depth, duration, and plan. Nitrox (EAN) blends, for example, increase the oxygen fraction to reduce nitrogen loading, which can help with longer bottom times at shallower depths. Trimix gets even more technical, mixing helium to keep oxygen and nitrogen partial pressures within safe bands while enabling deeper exploration.

In any case, the core idea stays: the air you breathe is a familiar trio, and the proportions matter because they determine how your body responds under pressure. This isn’t just textbook stuff; it’s a practical tool you carry with you every time you prepare a gas plan, review a gauge, or log a dive computer reading.

A few bite-sized, real-world takeaways

• Remember the rough composition: about 20% O2, 79% N2, 0.03% CO2. This helps you visualize what’s in the lungs when your regulator delivers air at depth.

• PPO2 matters. At the surface, air gives you a safe PPO2, and as you descend, you’ll hit new numbers. Good plan is to know acceptable PPO2 ranges and how depth changes them. Your dive computer or a gas calculator is there to help you stay in the safe zone.

• Nitrogen isn’t just filler. It’s the main inert gas that affects narcotic potential and decompression planning. Understanding its behavior helps you recognize why ascent rates and decompression stops exist.

• CO2 isn’t a nuisance; it’s a signal. If you’re breathing hard and your vents aren’t removing CO2 efficiently, you’ll notice fatigue or discomfort. Breath control and good ventilation become part of your safety toolkit.

• Gas blends are more than a gimmick. If you ever move beyond air, you’ll be juggling oxygen, nitrogen, and sometimes helium. The same core idea applies: know the percentages, know the pressures, and keep your limits in sight.

A tiny tour of the practical equipment and terms you’ll encounter

• Dive computers and gas plans: modern computers track depth and time, and most support multiple gas mixes. They’ll flag PPO2 limits if you’re mixing gases and staying at depth too long.

• Gas calculators and tables: handy tools for calculating how long you can stay at a given depth on a particular mix, without spiking the risk of oxygen toxicity or decompression requirements.

• Cylinders and labeling: standard air is widely used, but certified mixes come with specific labeling and handling rules. When you see a mix labeled with a higher oxygen fraction, you know the plan is different—sometimes a lot different.

A friendly digression: altitude and air composition

If you’re curious about where these numbers come from in different environments, consider altitude. At higher elevations, the air pressure is lower, so the partial pressures of the gases shift even though the percentages stay the same. That’s a reminder that “air composition” is a bit of a constant but the pressure context changes everything when you’re planning any exposure—whether on land or in the water.

Bringing it together: what to carry in your head and your kit

• Know the three main gases and their approximate percentages.

• Understand that depth changes the partial pressures you experience, and that safety limits exist for PPO2.

• Appreciate that nitrogen has real effects, even though it’s not “used” for metabolism in the same way oxygen is.

• Respect CO2 as a signal of ventilation and effort; keep it within comfortable levels.

• Be comfortable with gas planning tools, and don’t hesitate to consult your instructor or a trusted resource when you’re working with anything beyond standard air.

Final thought

Air is a tiny, everyday miracle—a blend of elements you hardly notice until you’re in a different element altogether. The same simple trio—O2, N2, and CO2—powers every breath, whether you’re lounging at a pool, cruising over a reef, or exploring a sunken wreck. Knowing their approximate mix isn’t just trivia; it’s peace of mind. It’s the quiet confidence that you’re carrying the right gas blend in the right amount, at the right time.

If you’re ever torn between memory and practicality, remember this: oxygen keeps you alive, nitrogen keeps you moving through the water, and carbon dioxide tells you when you’re breathing too hard or not venting enough. With that balance in mind, you’re not just ready to surface safely—you’re ready to enjoy the wonder beneath the waves.